365daysDate: August 21, 2009

Title: Flying Around the Solar System


Podcaster: Nancy Atkinson, with Neil Mottinger from JPL

Description: Just how do you navigate a spacecraft to a specific destination in our solar system? Universe Today’s Nancy Atkinson talks with Neil Mottinger from JPL about his career of robotic spacecraft navigation for NASA.

Links: Nancy’s article in the Oswego Ledger-Sentinel about Neil Mottinger and his career at JPL.
The oblique view of Victoria Crater.
For more from Neil Mottinger and three other navigators, read the article “The Navigators: How We Fly Spacecraft Around the Solar System” on Universe Today

Bio: Nancy Atkinson is the senior editor for Universe Today, is on the production team for Astronomy Cast, and is part of the IYA New Media Working Group, helping to bring the 365 Days of Astronomy podcast to you every day of 2009. She also is a NASA/JPL Solar System Ambassador

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by “The Siliconopolitan.” Be sure to read Kris Straub’s webcomic “Starslip,” a completely accurate portrayal of the future. At


Nancy: Navigating a spacecraft through the heavens has been compared to sailing a ship on the open seas or driving a vehicle on a long, cross country journey. Analogies are necessary, since spacecraft navigation involves some very high power math, and most of the time the navigators need to figure out how to do things that have never been done before. Those of us who have trouble making sense of a road map here on Earth stand in awe of what these celestial navigators can accomplish.

In simplest terms, spacecraft navigation entails determining where the spacecraft is and keeping it on course to the desired destination. But it’s not as easy as just getting from Point A to Point B. If Point A is Earth and Point B is a planet or other body in our solar system, the problem is that these are not fixed positions in space. Navigators must meet the challenges of calculating the exact speeds and orientations of a rotating Earth, a rotating target destination, as well as a moving spacecraft, while all are simultaneously traveling in their own orbits around the Sun.

Confused yet? I sure am, but I recently had the chance to talk with navigator Neil Mottinger, who has worked at the Jet Propulsion Laboratory since 1968. He’s been involved with missions like Mariner, Voyager, the Mars Exploration Rovers, the Mars Reconnaissance Orbiter, and many other international missions. So tell us Neil, just how do you do spacecraft navigation?

Neil: It all starts here on Earth. The majority of the traditional tracking has been radiometric tracking observations of the spacecraft velocity and range from tracking stations all over the world. And since we’re sitting on an Earth that rotates there is a definite signature impressed in the line of sight or Doppler data that we receive from the spacecraft and if we have really accurate information about where the tracking stations are on the Earth, and what the orientation of the Earth is, which means very precise knowledge of Universal Time, which tells you where the Earth is pointing, we’re able to infer the declination and right ascension of the spacecraft at a given time and once we’ve determined the trajectory from tracking data of where it has been and assuming models Newtonian acceleration due to the planets and the sun we are able to propagate forward.

Nancy: How have things changed over the years for navigation?

Neil: It’s more than just arriving at the planet at a precise spot, now we have to find ways to model our descent and control our descent through the atmosphere so that we’re able to come down in places that are safe, yet scientifically rewarding at the same time. Mars’ atmosphere — since we live in an atmosphere, we know how unpredictable the weather can be and its perhaps even more complicated when you are that far away and trying to anticipate how the winds will be blowing. Of course it all has to be compensated for autonomously, which we have seen happen with the MER landers, especially the second lander. I think there’s conclusive evidence that the terminal guidance system was able to fire the rockets to compensate for winds such that there was essentially no horizontal velocity when the lander hit the ground.

The changes I’ve seen in the decades I’ve been at JPL working on this, it was oversimplified it seems like, in the very beginning, if we look at maybe the Mariner 2 mission perhaps, the requirements were which side of the planet you might fly on. And as the missions progressed and we were able to improve our accuracy with the ground based navigation that I just described the uncertainty of the targeting points became smaller and smaller. They would want to fly by specific latitudes of planets perhaps.

Nancy: What lessons have you learned in your years of being a navigator?

Neil: You have to be very patient in this business. I remember I was closely associated with the Mariner 9 navigation team and when we got to Mars and got into orbit Mars celebrated our arrival with by throwing a total dust storm and the complete surface of the planet was obscured by dust, so all the planning that had been in place went on hold while we waited for the dust to subside. I remember some of the pictures that were taken, there were some bumps that began to appear, poking through the dust. They turned out to be volcanoes; I believe it was the Tharsis Ridge with Olympus Mons being the largest one. Eventually when the dust did settle we had pictures of all the volcanoes and the grand canyon, tremendous varieties of geologic formations on the surface of Mars. Obviously you have to be there a long time to take a lot of pictures.

Nancy: And now the spacecraft you are working with have progressed from not only taking pictures of the planet but also finding and taking pictures of other spacecraft on Mars.

Neil: That’s right, in fact I saw pictures just this week of an oblique view of Victoria Crater where Opportunity spent a year or two circling around the edge of that and they can see wheel tracks around in the sand around the crater. I think one of high points of the reconnaissance from that Mars orbiter has been the characterizations of small areas of the surface. We’re able to point instruments with higher and higher resolution. We’re able to control the orbit of the spacecraft so that scientists can be very specific in where they want to point the instruments.

Nancy: Can you talk about some of the instances where navigation has really made a big impact on a mission?

Neil: Probably one of the most spectacular ones there was the Voyager flyby of Jupiter. Regular navigation as I’ve described initially where you’re just sitting on the Earth making measurements of the line of sight velocity and range to the spacecraft to a reasonably good job of perhaps determining the right ascension of the spacecraft, but it is much more difficult to get the declination or the transverse position of the spacecraft so techniques were developed over the decades to incorporate optical navigation where you use the cameras on the spacecraft to take pictures of not only the target body or perhaps a satellite. In the case of Jupiter you’ve got four Galilean satellites you can photograph and you can use those images taken from the spacecraft to help resolve a component of where the spacecraft is that can’t be done from the ground based tracking or certainly can’t be done with short tracking from ground based.

It was in the process of an optical navigator in our section, Linda Morabito, going back through the photographs that had been taken for optical navigation and essentially making her final report, and reanalyzing the optical data and combining it with the radio that she discovered there was something more than just the surface of Io in the pictures she had been using. Maybe it looked like an umbrella or a lawn sprinkler. The ultimate revelation was that she saw an active volcano from the surface of Io. This was the first case of active volcanism anywhere other than the surface of Earth, and I’ll have to confess to a lot of pride that that lady as a member of the navigation team was the one to find that. And then there’s a tremendous scurry to look at previous pictures of Io in a new light, and any spacecraft since that has spent any time around Jupiter, which has been certainly Galileo spacecraft but even flybys of New Horizons on its way to Pluto, they all want to take pictures of Io. We’re discovering there is continuous volcanic activity going on there, continuously resurfacing the surface of that satellite.

Nancy: Neil, you and I have talked before, and in fact I’ve written an article about your career and I’ll put a link to that in the show notes, but what has your time at JPL as a navigator meant to you?

Neil: It’s been fulfilling beyond my wildest expectations. I grew up on a small family farm in the Midwest and never dreamed that I was going to end up in the middle of the unmanned space program at the Jet Propulsion Laboratory. I probably didn’t have a career plan other than trying to work hard and hopefully have something to show for my life when I was done. The opportunities opened up to come and work at JPL, and it certainly wasn’t because of outstanding academic performance, but I was just trying to work hard and do a good job. Sometimes I think the educational setbacks along the way were the best thing that ever could have happened. I consider it an incredible honor to work with what I consider some of the most gifted navigators in the world as we go about exploring our solar system.

End of podcast:

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